Removable pressure sensitive adhesives (PSA) adhered to a backing, which predictably adhere, yet remain repeatedly peelable from a variety of substrates over a long period of time without damaging or marring the substrate, have many commercial uses. For example, masking tapes, removable labels or office notes, protective films and medical tapes all must quickly adhere to metal, paper, plastics and skin, respectively, but must also peel smoothly away from these varied substrates without leaving behind any adhesive residue on or damaging the surface of a particular substrate.
Ideally, depending on the substrate, the removable adhesive must provide sufficient tack (or quick stick) to quickly fix the adhesive to the desired substrate, adequate peel strength to prevent damage of the surface when the adhesive is removed, and have the appropriate cohesive strength to control the transfer of adhesive to the substrate. Cohesive strength must also be controlled in order to limit the cold flow of the adhesive on a surface, a process which leads to an undesirable building of peel strength over time. Balancing these pressure sensitive adhesive properties, particularly in a removable adhesive, poses difficulties to the formulator.
U.S. Pat. No. 3,691,140 (Silver) discloses the use of solid, inherently tacky, pressure-sensitive adhesive microspheres which, when adhered to a first substrate, may when contacted to a second substrate be easily removed from the second substrate without delaminating the first substrate or the second substrate. While exhibiting a high degree of removability, microsphere adhesives tend to transfer to contact surfaces such as the second substrate.
U.S. Pat. No. 5,045,569 (Delgado) discloses hollow, inherently tacky pressure-sensitive adhesive acrylate microspheres which, due to their unique morphology, provide enhanced peel and shear strength and less adhesive transfer to substrates than "repositionable" adhesives based on solid acrylate-based microspheres.
U.S. Pat. No. 4,599,265 (Esmay) discloses an adhesive having a high degree of cohesive strength. The adhesive disclosed is an acrylate, has low tack and maintains peelability from a variety of ordinary substrates. Esmay teaches that through the crosslinking of the tape's adhesive layer and the use of low levels of polar monomer (up to 3 mole percent of a strongly polar monomer, such as acrylic acid) along with alkyl acrylates having side chains 4-12 carbons in length in the copolymeric adhesive, the required balance of low tack and high cohesive strength can be imparted to the removable adhesive. One possible drawback associated with the Esmay adhesive is that its monomers may bleed through a substrate such as paper prior to curing. Such bleeding is undesirable because of the damage done to the substrate.
U.S. Pat. No. 4,737,559 (Kellen et al.) describes a PSA formulation in which the viscous flow, and resultant adhesion buildup, is controlled through the addition of a small amount of a free radically polymerizable photocrosslinker (a mono-ethylenically unsaturated aromatic ketone, most preferably 4-acryloyl-oxy-benzophenone) to at least one alkyl acrylate or methacrylate. The Kellen et al. adhesive is formulated for adhesion to skin and at times, considered to be too aggressive for paper.
U.S. Pat. No. 3,635,754 (Beede) discloses the use of a heat-activated, multi-phase pressure-sensitive adhesive copolymer whose melting point is greater than 35.degree. C. and whose glass transition temperature (T.sub.g) is below 5.degree. C. At room temperature, the adhesive is non-tacky. At that temperature, a crystalline or ordered phase and an amorphous or disordered phase co-exist. At skin temperature (approximately 35.degree. C.), the copolymer becomes sufficiently amorphous to become tacky. This phenomenon may be reversed by cooling the adhesive for removal from a substrate, with the crystallization of the side chains providing sufficient cohesive strength to prevent any transfer of adhesive to the substrate.
World Patent Application Ser. No. 90/13420 (Stewart et al.) discloses a temperature activated, pressure-sensitive adhesive which is rapidly converted from non-tacky to tacky over a predetermined temperature range. Similar to the multi-phase adhesive of Beede, this temperature dependent adhesive relies on the careful selection of side chain crystallizable monomers, particularly those monomers which have linear aliphatic side chains of at least 10 carbon atoms, including C.sub.14 -C.sub.22 alkyl acrylates or methacrylates. Stewart et al. specify that these compositions, when a mixture of copolymerizable monomers are used, must contain at least 50% by weight monomers with crystallizable side chains. However, at room temperature, these adhesives are substantially non-tacky.
U.S. Pat. No. 4,038,454 (Lehmann et al.) discloses a pressure-sensitive adhesive which comprises either a primary alkyl acrylate or a mixture of primary alkyl acrylates and crosslinker. Lehmann et al.'s alkyl acrylate contains at least four carbon atoms. If a mixture of primary alkyl acrylates is utilized, the primary alkyl acrylates can have carbon atoms which contain 1 to 14 carbon atoms. Half the carbon atoms should contain at least four carbon atoms in a mixture.
In using PSAs, it is also desirable to combine these removable qualities with the procedural, economical and environmental advantages of a hot melt processible adhesive. To date, most attempts to prepare acrylic hot melt PSAs have been limited because such materials have poor cohesive strength at room temperature or are too viscous even for use at high temperatures found in current melt processing equipment and practices. Maintaining sufficient cohesive strength, as noted above, is one critical factor in the formulation of permanently removable PSAs.
Typically, the cohesive strength of acrylic polymeric PSAs, which are not microspheres, may be improved in a variety of ways, but each way has its disadvantages, especially when the formulator wishes to maintain a low melt viscosity for the PSA. The molecular weight of the acrylic polymer may be raised to improve cohesive strength, but an unavoidable and unacceptable rise in melt viscosity also results. Polar monomer content, such as acrylic acid, may also be increased to improve cohesive strength, a property which is useful in many adhesive applications. However, increased polar monomer content has caused greater adhesion buildup, as described by U.S. Pat. No. 3,008,850 (Ulrich). Finally, cohesive strength may be enhanced through the covalent crosslinking of the adhesive. Covalent crosslinking, though providing the needed increase in cohesive strength, eliminates the potential for melt processing.
Physical crosslinking is described in U.S. Pat. No. 4,554,324 (Husman et al.). Husman et al. disclose a hot melt processible acrylate PSA which gains the needed balance of cohesive strength, high tack, and low melt viscosity through the chemical modification of the soft acrylate backbone by grafting reinforcing high polymeric moieties to the acrylate chain. These high moieties provide glassy domains which enhance the cohesive strength of the adhesive at lower temperatures without dramatically increasing the melt viscosity of the composition. The peel adhesion strengths of the exemplified compounds, however, tend to exceed those values which are required for removability from most substrates.
Thus, there currently exists a need for a removable acrylic pressure sensitive adhesive which displays a superior balance of tack, peel strength and cohesive strength at room temperature and which may be formulated for removal from a variety of substrates without damaging or depositing adhesive residue on the substrate and without building excessively in adhesion over time.